1. Field of the Invention
The present invention relates to an electrophotographic image forming method.
2. Description of the Background
In electrophotographic image forming apparatuses, seamless belts are used with various functions and uses, such as a paper conveyance belt.
Recently, seamless belts are also used as an intermediate transfer belt in full-color electrophotographic image forming apparatuses. The intermediate transfer belt is a member onto which four or more toner images of different colors are transferred from one or more photoreceptors to form a composite full-color toner image thereon. The composite full-color toner image is then transferred from the intermediate transfer belt onto a transfer medium such as paper.
Demands for intermediate transfer belt are growing in accordance with recent colorization of copiers. The intermediate transfer belt is generally comprised of materials such as thermoplastic resins, thermosetting resins, rubbers, and elastomers. The intermediate transfer belt is widely used in electrophotographic image forming apparatuses in which developing devices are arranged in tandem (hereinafter “tandem electrophotographic image forming apparatuses”) for the purpose of increasing printing speed.
In the tandem electrophotographic image forming apparatus, the intermediate transfer belt is required not to deform through an image forming operation so as not to cause color drift in the composite full-color toner image. The intermediate transfer belt is also required to be strong and durable enough to withstand repeated use. Because the intermediate transfer belt is also required to be flame-resistant, polyimide resins and polyamide-imide resins have been preferably used therefor. In particular, polyimide resins are more advantageous from the viewpoint of creep deformation property, durability, and controllability of electric properties.
In operation, such intermediate transfer belts are susceptible to two phenomena, abrasion and filming. These are described below.
In an image forming operation, some toner particles may remain on the intermediate transfer belt after image transfer without being transferred onto a transfer medium. Therefore, it is necessary to remove such residual toner particles from the intermediate transfer belt. One of the most effective ways to remove residual toner particles from the intermediate transfer belt is to scrape the residual toner particles off by pressing an edge of a cleaning blade against the surface of the intermediate transfer belt.
On the other hand, the intermediate transfer belt is provided with electric properties, required when transferring toner images. However, the electric properties deteriorate with repeated image forming operations, because the cleaning blade is constantly pressed against the intermediate transfer belt. Thus, the lifespan of the intermediate transfer belt is shortened and eventually abnormal images are produced.
Moreover, in accordance with the widespread use of oilless fixing systems, recent toners typically include waxes and soft materials, which are easily softened, for the purpose of reducing fixable temperature to save energy. Such waxes and soft materials are likely to adhere to the intermediate transfer belt and to be gradually formed into a film with repeated image forming operation. This phenomenon is hereinafter referred to as filming. The electric properties of the intermediate transfer belt deteriorate due to the occurrence of filming.
Additionally, demand for forming images not only on typical copying paper but also on coated paper and printing paper that include large amounts of loading materials such as talc, kaolin, and calcium carbonate, is increasing. Such loading materials are likely to adhere to the intermediate transfer belt when images are transferred from the intermediate transfer belt onto such types of paper, and the adhered loading materials are likely to cause filming.
In attempting to suppress both the damage caused by pressure from the cleaning blade and the occurrence of filming caused by toner component materials and loading materials, one proposed approach involves reducing the surface friction coefficient of the intermediate transfer belt. For example, Japanese Patent Application Publication No. (hereinafter “JP-A”) H08-95455 and Japanese Patent No. 3753909 each propose applying a metal soap (i.e., a lubricant) to the surface of an intermediate transfer belt to reduce the surface friction coefficient thereof.
An image forming apparatus in which a metal soap is applied to the intermediate transfer belt is able to form high quality images at low printing speeds. However, when the printing speed is high, as is consistent with recent demand, abnormal images with undesired stripes may be formed in continuous image forming operations. This is because a portion of the intermediate transfer belt on which an image exists has a higher surface friction coefficient than a portion on which no image exists, and therefore toner component materials and loading materials are selectively adhere to the portion having a higher surface friction coefficient on which an image exists. Thus, the electric properties of the portion to which toner component materials and loading materials are adhered deteriorate, resulting in production of abnormal images with undesired strips.
In attempting to apply a sufficient amount of a lubricant to an intermediate transfer belt, one proposed approach involves controlling a surface profile of the intermediate transfer belt. For example, JP-2005-316231-A and JP-2004-361694-A control the surface profile of an intermediate transfer belt by roughening its surface with an abrasive paper or an abrasive agent.
The inventors of the present invention attempted to optimize the surface profile of an intermediate transfer belt by referring to JP-2005-316231-A and JP-2004-361694-A. However, even in a case where a sufficient amount of a metal soap was adhered to the intermediate transfer belt, filming occurred in some portions on the intermediate transfer belt when images were formed at high speeds. Observation of this intermediate transfer belt using an electron microscope revealed that a portion where filming occurred had only a very small amount of the metal soap.
Accordingly, to form high quality images even when the printing speed is high, the metal soap is required to be applied to the whole surface of the intermediate transfer belt, which has never been achieved.
Zinc stearate is a widely-used metal soap as a lubricant for an intermediate transfer belt. It is well known that the coverage of zinc stearate on an intermediate transfer belt can be measured by X-ray photoelectron spectroscopy (XPS), as described in JP-2005-17469-A, JP-2005-249901-A, JP-2005-004051-A, and JP-2004-198662-A.
XPS detects all elements other than hydrogen, existing on the outermost surface of a sample. Consider a case where a photoreceptor to which zinc stearate is applied to its surface is measured by XPS. As the coverage of zinc stearate increases, the measured elemental composition approaches from that of the photoreceptor alone to that of zinc stearate alone. When the coverage of zinc stearate is 100%, the measured elemental composition becomes theoretically equivalent to that of zinc stearate, and the measured amount of zinc becomes saturated. Specifically, the saturation amount of zinc is 2.44% by atom, which is theoretically calculated from the elemental composition of zinc stearate. Thus, the coverage of zinc stearate can be measured from the following formula:(Zn/2.44)×100wherein Zn (% by atom) is the amount of Zn of the sample measured by XPS.
It is more advantageous to mix zinc stearate with a certain amount of zinc palmitate. Because zinc palmitate has a lower melting point than zinc stearate, the mixture of zinc stearate and zinc palmitate can be more easily spread over the intermediate transfer belt even when the linear speed is high.
However, the theoretical saturation amount of zinc, i.e., 2.44% by atom, which is theoretically calculated from the elemental composition of zinc stearate, cannot be used for calculation of the coverage of the mixture of zinc stearate and zinc palmitate. The theoretical saturation amount of zinc has to be recalculated for each mixing ratio of zinc stearate and zinc palmitate, which is less practical and economical, and more cumbersome.
To solve these problems, a method for calculating the coverage of the mixture of zinc stearate and zinc palmitate on the surface of an intermediate transfer belt, regardless of their mixing ratio, is herein proposed. This method is applicable to a case where any metal soap, not limited to the mixture of zinc stearate and zinc palmitate, is applied to an intermediate transfer belt including nitrogen on its surface. Specifically, this method calculates the coverage of a metal soap on an intermediate transfer belt from the rate of decrease in the percentage content of nitrogen at the surface of the intermediate transfer belt before and after applying the metal soap thereto. More specifically, the exposure rate of the intermediate transfer belt is calculated from the above rate of decrease in the percentage content of nitrogen at the surface of the intermediate transfer belt, and then the coverage of the metal soap is calculated from the following formula:Coverage(%)=100−Exposure rate(%)
When the exposure rate thus calculated before any given image forming operation is 30% or less, more preferably 20% or less, and most preferably 10% or less, high quality images can be produced for an extended period of time.
However, even when the exposure rate thus calculated before any given image forming operation is 30% or less, blurred images are found to be produced under high-temperature and high-humidity conditions in some cases. As a result of detailed analysis of these cases, toner components (e.g., silica, wax) or loading materials in paper are found to be adhered to the portion where the blurred images are produced. Accordingly, the coverage of contaminants (e.g., toner components, loading materials in paper) after the image formation should be taken into consideration, as well as the coverage of the metal soap, when calculating the exposure rate of the intermediate transfer belt.